Nuclear Seminar

R-process Abundance Patterns and
the Existence of Shell Structure

Dr. Sean Liddick
UNIRIB, Oak Ridge National Laboratory

*Thursday, March 23, 2006   4:30 p.m.   NSH 124
(Refreshments served prior to seminar in NSH 124)

 

The observed peaks in the r-process abundance pattern is a dramatic fingerprint of a fundamental aspect of nuclear structure: shell closures and the underlying single-particle structure of nuclear systems. However, until recently, knowledge of shell closures was derived mostly from studies on stable isotopes. It was long assumed that shell closures would not change and were treated as constants as a progression was made towards more exotic nuclei. Surprisingly, in recent years it has been observed that the shell closures are not necessarily fixed across the nuclear landscape and the alteration may have dramatic impacts for the creation of the elements. While heavier nuclei along the r-process are still just beyond our experimental reach, studies of lighter mass systems could provide insight into the mechanisms driving the evolution of shell structure.

One such mechanism for altering the relative ordering of single-particle states, thus leading to the evolution of new shell structure, is an attractive proton-neutron monopole interaction between spin-orbit partners. A wealth of new experimental data on the beta-decay properties and low-energy level structure of neutron-rich A~60 pf-shell nuclei have been obtained at the National Superconducting Cyclotron Laboratory using the Beta Counting System (BCS) allowing for a systematic investigation of the pf7/2 - nf5/2 monopole interaction. Fast beams of secondary reaction products resulting from the bombardment of a 140 MeV/A ⁸⁶Kr primary beam with a thick Be target were selected using the A1900 fragment separator and implanted into a 1500 mm thick double-sided silicon microstrip detector at the center of the BCS. Prompt and delayed gamma-ray transitions were detected using 12 high-purity germanium detectors from the MSU Segmented Germanium Array.

The new beta-decay results were compared to predictions of shell model calculations. In many cases, the beta-decay properties and the low-energy excited states in the daughter nuclei, determined by beta-gamma coincidences, are well reproduced by the shell model calculations. The results have improved our understanding of the dynamic nature of the neutron single-particle states in this region due to the strong, attractive pf7/2 - nf5/2 monopole interaction, leading to the development of new shell structure far from stability.

*Note different day and time.